Work machines, including on and off-highway haul and vocational trucks, wheel loaders, motor graders, and other types of heavy machinery generally include a multi-speed, bidirectional, automatic transmission drivingly coupled to an engine by way of a hydraulic torque converter. The hydraulic torque converter multiplies and/or absorbs torque fluctuations transmitted from the engine to the transmission by allowing slippage between an output shaft of the engine and an input shaft of the engine. To improve fuel consumption and reduce power loss due to the slippage, the torque converter typically includes a lockup clutch for mechanically coupling the engine output and transmission input shafts.
When the engine output and transmission input shafts are mechanically coupled, the engine can be used to slow the work machine's travel. For example, power can be transferred from the wheels of the work machine in reverse direction through the transmission to drive the mechanically coupled engine. The natural resistance of the engine then dissipates some of the transferred power, thereby slowing the work machine. Additional power may be dissipated by the use of engine or exhaust braking that increases the resistance of the engine. However, because typical torque converters disengage the lockup clutch at low vehicle speeds to minimize the likelihood of stalling the engine, the retarding capacity of the work machine may be greatly reduced at these low vehicle speeds. That is, little or no power may be transferred from the wheels to the engine when the lockup clutch is disengaged and slippage is allowed to occur.
One method of improving the retarding capacity of a work machine at low travel speeds is described in U.S. Pat. No. 6,152,853 (the '853 patent) issued to Banks, III Nov. 28, 2000. The '853 patent describes an exhaust braking system that includes a vehicle speed sensor, an exhaust brake valve, and a controller in communication with a lockup clutch and a transmission. If a sensed vehicle speed is greater than a speed value preset by an operator via a user interface, the controller engages the lockup clutch and closes the exhaust brake valve to bring the vehicle speed back down to the preset speed value. If it is determined that the vehicle speed is greater than the preset speed value and increasing, additional retarding may be implemented by downshifting the transmission.
Although the exhaust braking system of the '853 patent may sufficiently control the travel speed of a vehicle, it may be inadequate for some situations and excessive for others. In particular, because the exhaust braking system of the '853 patent only engages when the vehicle speed exceeds the operator preset speed value, it may do nothing to retard the vehicle when the vehicle operates at speeds lower than the preset speed value, even if engine retarding is desired. In addition, because the lockup clutch only engages when the exhaust brake valve is activated, lower levels of retarding that require only the natural resistance of the engine may not be possible.
The disclosed retarding system is directed to overcoming one or more of the problems set forth above.